WAP54G Ver 3 Speed Drop Problem

hi , i set up One WAp54G Ver 2 , with one Ver 3 . Bridge to each other with 25 Db Directional Antenna , the speed raise up to 17 ~ 20 , but suddenly goes and stop on 0 !!!!
when i ping the devices , from each other , have many Time Out , 8 in 100 ( 8 % )

PS : also have another Ver 2 , with this one bridge to the other Ver 2 , the speed doesn't drop ,
the Ver 2 device is using 3.04 , and the Ver 3 too , also tried the 3.05 on ver 3 )
Thanks.